This invention relates to a method for operating a laser for an interstitial thermotherapeutic treatment of biological tissue in which, for producing a coagulation necrosis, the radiation of the laser is guided into the tissue within a radiation period by way of a fiber optical waveguide.
In interstitial thermotherapy by means of laser radiation, a special-construction fiber optical waveguide (for example, according to European Patent Document EP 0 292 695 B1) is guided into the tissue to be heated. In this case, the application of the laser radiation causes a thermal denatuation of the tissue and therefore the formation of a coagulation necrosis. On the one hand, the effect of the applied laser radiation is determined by the laser parameters, such as the wave length, the duration of the radiation, the laser power and the power density and, on the other hand, by the tissue parameters, such as the absorption capacity, the scattering, the heat conduction, the tissue structure and the blood supply.
At temperatures of above 60.degree. C., the tissue parameters will change, for example, as a result of coagulation, carbonization or closure of the blood vessels. Particularly, the tissue carbonization, for example, as the result of excessive laser power or excessive power density, leads to a drastic increase of the absorption capacity, whereby a deep penetration of the photons into the tissue is prevented. In addition, a carbonization of the tissue in the immediate surroundings of the fiber optic waveguide leads to an abruptly increased thermal stress of the fiber optic waveguide and may therefore cause its destruction. For this reason, up to now, the laser power has been adjusted to a value which is far below the destruction threshold for the fiber optic waveguide so that, on the one hand, it is ensured that neither the tissue will be carbonized during the radiation period, nor the fiber optic waveguide will be overloaded but that, on the other hand, a coagulation necrosis will be created which is sufficiently high.
However, for example, in the treatment of benign hyperplasia of the prostate, this has resulted in radiation periods of 10 minutes per puncture respectively in which case, on the average, 8 punctures are required for a treatment. In the case of the known method, this results in a total treatment duration of approximately 2 hours.
It is an object of the present invention to provide a method for operating a laser for an interstitial thermotherapeutic treatment of biological tissue which, while the above-mentioned boundary conditions are maintained, particularly for avoiding a tissue carbonization and a destruction of the fiber optic waveguide, permits a considerably shorter treatment duration.
This object is achieved by a method for operating a laser for an interstitial thermotherapeutic treatment of biological tissue in which, for generating a coagulation necrosis, the radiation of the laser is guided into the tissue by way of a fiber optic waveguide within a radiation period, characterized in that, during the radiation period, the laser power is reduced in a time-controlled manner continuously or in steps.
By means of a controlled continuous or step-by-step reduction of the laser power during a radiation period after the fiber optic waveguide punctures the tissue, the invention takes into account the complex occurrence of the heat conduction at the transition between the fiber optic waveguide and the tissue, which is present particularly in the case of the interstitial treatment by means of fiber optic waveguides. In the case of the initially cold tissue, a much higher laser power may be applied for a short time until the carbonization temperature has been reached. Since, by means of the heating of the tissue by way of the applied laser radiation, a heat transfer takes place at the same time by way of the surrounding tissue, which, however, is continuously reduced as the heating increases, the laser power is reduced continuously or step-by-step during the radiation period from the value, which initially is limited only by the maximum power of the laser or of the destruction threshold of the light-guiding components, such that the carbonization temperature of the tissue is not exceeded in the radiation period. Thus, the radiation duration for generating a coagulation necrosis of an equal size is reduced to approximately half the previous radiation time.
In contrast to measures involving control techniques, the power reduction of the laser, which takes place in a controlled manner, can be implemented without any problem by means of the known lasers provided for medical applications.
Furthermore, the method according to the invention has the effect that blood vessels situated in the surroundings of the used fiber optic waveguide are closed considerably faster and therefore a clear reduction of the heat removal is achieved by convection. The coagulation temperature at the transition between the fiber optic waveguide and the tissue is also achieved much more rapidly.
The optimal control curve for the reduction of the laser power is mainly a function of the tissue and is advantageously determined empirically. For most treatment cases, an initial laser power of maximally 25 watts with a continuous reduction during the radiation period to minimally 5 watts was found to be useful.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.